Active crossover

Any speaker with a passive crossover can be turned into an active system by removing the passive crossover.
You need a binding post per driver, an amp per driver and of course, an active crossover.
Doubling your amplifier budget might not appeal to you except if you believe in bi-amping.
There are voices stating that you are not in need of doubling.
Active crossovers make amplifier live much easier, no need to buy very expensive ones.
But I must admit these voices are often stakeholders too.

Eliminating the Need for Expensive Electronics and Cables
The CS2.3 is a very accurate reproducer of sound. Many speakers, due to their designs, require the need for very expensive electronics to achieve satisfactory performance. Conventional designs have problems with the impedance of the drivers due to mechanical and electrical resonances and the passive crossovers used. As mentioned earlier, passive crossovers are power-robbing components. The CS2.3 maximizes power transfer. With bi-amping and the 48 dB slope of the digital crossover, the listener utilizes all of the power available from the amplifier. The high frequencies are not harsh or strident and therefore, the need for a softer sounding tube amplifier is not required for the tweeters. The need for expensive cables that filter out high frequency information is also not required. The need for 200 pound amplifiers with huge power transformers is not necessary since the CS2.3 is very efficient and does not require the large amounts of power that expensive amplifiers offer into low impedances. In fact, you may find that you will spend hundreds of dollars per channel rather than thousands of dollars per channel to drive the CS2.3.
Emerald Physics

Active crossovers targeting the audiophile are rare just like active speakers are rare.
The pro-world has more to offer but audiophiles in general think pro-gear is below par.

Emerald Physics is a nice example, they combine their speakers with Behringer pro-gear.

Active crossovers can be implemented using an analogue circuit or using DSP.

Analogue

Bryston 10B $3050

The Model 10B features independently selectable crossover points for high-pass and low-pass, in case the speaker installation requires slightly overlapped, (or slightly staggered), response curves for the drivers. You can also independently select crossover slope, from 6, 12, or 18dB/Oct., where one driver requires faster cutoff than another in the same system. A fixed frequency plug in Linkwitz-Riley alignment with steep rolloff curves of 24 dB/Oct. is also available.

Rane AC 22S Active Crossover $ 449

Stereo 2-Way, or Mono 3-Way
Linkwitz-Riley Alignment with 24 dB per Octave Slopes
Adjustable Driver Alignment Delays
Infrasonic, Ultrasonic and RFI Filters
Low Output Muting (2-Way)
Mono Sub Output Option
Constant-Directivity Horn EQ Option
Input & Output Level Controls
Fully Active Balanced XLR Inputs & Outputs
Internal Universal Power Supply (100-240 VAC)

DSP

DSP is using a processor to implements the crossover.

Behringer DCX2496 $ 439.99

3 analog inputs (one suitable as digital stereo AES/EBU input) and 6 analog outputs for maximum flexibility
High-end AKM® 24-bit/96 kHz A/D and D/A converters for ultimate signal integrity and wide dynamic range (113 dB)
Integrated sample rate converter for easy connection of external digital signals with sampling rates from 32 to 96 kHz
Precise dynamic EQs for level-dependent equalization and extremely musical parametric EQs, selectable for all inputs and outputs/
Several types of EQs (LP/BP/HP) for each input and output/
"Zero"-attack limiters on all output channels for optimal speaker protection/
Four different mono and stereo output operating modes
Individual crossover filter types (Butterworth, Bessel and Linkwitz-Riley) with selectable roll-off characteristics from 6 to 48 dB/octave
Adjustable delays for all inputs and outputs-allow manual or automatic correction for room temperature, phase and arrival time differences
Additional sum signal derived from the A/B/C inputs offers a fourth internal input signal
Future-proof ULTRADRIVE PRO software enables single or multi remote control via PC through RS-232 and RS-485 interfaces
Link option via RS-485 network interface enables cascading of several ULTRADRIVE PROs
60 user-defined internal presets storable via PC software or on PCMCIA card
Open architecture assures easy software updates
Servo-balanced, gold-plated XLR connectors for all inputs and outputs
High-quality components and exceptionally rugged construction for long life and durability
Internal switch-mode power supply for maximum flexibility (100 - 240 V~), noise-free audio, superior transient response plus lowest possible power consumption

DEQX HDP-3 Preamp proccesor AUD 6575

The HDP must be placed between the pre- and the power amp.

The signal from the pre-amp is converted to digital.

The DSP is applied and the result is converted by the DAC's.

Provides DEQX Calibrated™ speaker correction for passive speakers
Provides DEQX-HD™ speaker correction for up to 3-way active speakers
DEQX-HD™ active crossovers; 48dB/octave to 300dB/octave linear-phase
Traditional active 3-way crossovers from 6dB/octave to 48dB/octave
Frequency-response of speakers typically corrected from 6dB range to 1dB
Automatically corrects phase and group- delay over typically 3ms window
32-bit, floating-point, 240-MFLOPS processing with dual SHARC DSPs
Audiophile 24-bit 96kHz ADC-DSP-DAC processing and conversion
Provides anechoic measurement and detailed display for speaker correction
Provides room-measurement/display (non-anechoic) for room correction
Calibration Kit provides DEQX-calibrated measurement microphone
Two analog inputs: Unbalanced (2 x RCA) and Balanced (2 x XLR)
Two stereo digital inputs: S/PDIF (1 x RCA) and AES/EBU (1 x XLR)
Six analog outputs: unbalanced (6 x RCA)

HOLM Acoustics DSPre 1

Most active crossovers expect a analogue input.
If the crossover is done using DSP inevitably the analogue input has to be converted to digital (AD) first. Then the DSP is applied. The resulting signals are send to the DAC’s and converted to analogue again.

If your audio is digital you can save two DA – AD conversions if the active crossover uses DSP and accepts digital inputs. Holm is one of the very few I know applying the DSP straight on the digital input.
However it also accepts analogue inputs. They have to be AD'ed first of course.

DSPre design:
The DSPre unit is designed with the objective to have the least clutter in terms of controls.All other control of the quite advanced DSPre is done from the PC software DSPreLab.
DSPre software DSPreLab:
DSPreLab is the full featured PC application that transforms DSPre to a comprehensive speaker measurement tool and hold the power to transform these measurements into state of the art digital filters to run real-time on the DSPre. No PC needs to be connected during normal playback. The filter-type run on DSPre is FIR "Finite Impulse Filter" hence the name DSPreLab.
Room adaption
Individual filtering and correction of every driver
Perfect summing of driver outputs
Calibrated measurement microphone
Jitter removal from any digital source - "Clock Flawless" system
USB audio playback and recording of any input (adaptive 16 bit/48 kHz)
Digital inputs TOSLINK, SPDIF, AES/EBU:
Sample-rates from 11,025kHz up to 196kHz and are sample-rate converted to the internal 44,100 kHz processing rate.
Analog to Digital inputs natively, no extra cost for A/D modules:
DSPre do all the calculation digital domain and therefore analog input have to be converted into digital signals. Our preamplifier has two stereo Analog to Digital Converters for analog line signal. The ADC sensitivity can be adjusted to match the signal level of your analog source. ADC's convert to 24bit at 88,200 kHz Sample-rate.
6 analog outputs for stereo bass, mid and tweeter:
The output digital to analog converters are module based in a manner to comfort your system. For conventionally filtered speakers only one stereo DAC is used. For 2-way and 3-way systems a second and third DAC is mounted, one DAC output for every speaker unit. The DAC's are 24bit and are up-sampled to 352,800 kHz and anti-aliasing filtering is made outside the frequency band of the human ear, deemphasizing analog component performance. Volume control is achieved by lowering the DAC reference current for damping in the region 0 to -32dB, from -32dB and down the volume is adjusted inside the DAC's 24bit multiplier. This way a analog attenuator is omitted in the signal-path, and there is only a minor bit-resolution-penalty at very low listening-levels.
A model with digital out only is also available. This allows you to use the DAC's of your choice.
Band-less equalizer to tune to your preference:
The DSPreLab software includes a band-less equalizer who have more than 16000 individual frequency-discretions for setting up to 100 EQ points.

Valid Q1 2010
	Suggested retail
model:	€
DSPre 1, 1-way, Analog out	5.900
DSPre 1, 2-way, Analog out	6.900
DSPre 1, 3-way, Analog out	7.900

DSPre 1, 1-way, Digital out	5.400
DSPre 1, 2-way, Digital out	5.950
DSPre 1, 3-way, Digital out	6.500

Tact M/S 2150 XDM USD 4800.00

5 digital inputs and mic input
Level balancing and time alignment
Computerless digital crossover package
Full blown 12 band parametric EQ
2-channel digital amplification width full spectrum Dynamic Room Correction

A true digital amplifier, accepting PCM input and using PWM (Pulse Width Modulation) to amplify.
The M/S-2150 XDM crossover package is designed to support front panel crossover design, thus totally eliminating the need for computer interface. All crossover filter design algorithms are implemented in the M/S-2150 XDM internal processing unit. Crossover filters are designed and loaded in a split of a second for immediate listening and evaluation.
Supported crossover filters are Butterworth and Butterworth squared

All filter cutoff frequencies can be set anywhere between 10 and 24 KHz with a resolution of 1 Hz and with filter slopes from 6 to 72 dB/octave.

As it is a 2 channel amp you can only use this with 2 way speakers.

You need 2 amps to drive a 2 channel 2 way speaker setup.

Amps can be synchronised.

PC

A multi channel soundcard and the appropriate software allows you to do the DSP on a PC.

Thuneau - The Frequency Allocator

$149.00

The Loudspeaker Frequency Allocator is a 4-way, stereo DSP loudspeaker crossover that works with ASIO compatible multichannel sound card. Layout of each crossover leg is consistent with the way professional loudspeaker design software packages work. User is not limited to pre-canned filter algorithms such as B3 or LR4 that are so prevalent in the available hardware and software crossovers on the market today. Instead, we provide a series of flexible processing blocks that can be individually optimized to deliver the best match for the loudspeaker drivers used. Textbook filter settings are of course possible (either through direct input or by recalling factory patches), as are a vast range of other settings.
A brief list of features:

fully integrated Phase Arbitrator section for transient-perfect reproduction.
each DSP crossover leg adjustable from 6dB/oct to 42dB/oct on Low Pass and High Pass side.
4- band fully parametric equalizer in each leg.
low and high shelving filters in each leg- both switchable between 6dB/oct and 12dB/oct (with q adjustment).
up to 50msec delay in each leg- adjustable in 0.01msec steps (3.4mm resolution).
output polarity switch in each leg.
filter response display.
frequency response data (.frd text file) for each loudspeaker driver can be imported and merged with filter's response.
three zoom levels for viewing of the graphs.
fully skinnable- GUI clearly laid out in an xml-like file.
comes with two premade skins.
64bit processing throughout. SSE2 instructions utilized.

Source: Thuneau

DIY

MiniDSP $99

Item	Description
Digital Signal processing Engine	28/56 bit Digital Processor - Double precision processing
Host processor	48MHz micro-controller
Sample rate	48/96/192kHz (depending on the plug-in configuration )
ADC/DAC resolution	24 bits
Analog audio input	Unbalanced signal, RCA terminated, 2 x Inputs
Maximum input level, THD<1%	0.9Vrms (RevA) or 2Vrms (RevB)
Dynamic range, un-weighted	>98dB
Input impedance	6kΩ
Analog audio output	Unbalanced signal, RCA terminated, 4 x Outputs